java 電卓作成

Javaで電卓を作成しています。以下のコードのkeyPanelのところをJButtonに変更するにはどうすればいいでしょうか？

import javax.swing.*;
import java.awt.event.*;
import java.awt.Container;
import java.awt.BorderLayout;
import java.awt.FlowLayout;
import java.awt.GridLayout;

class CalcSample extends JFrame{
private static final long serialVersionUID = 1L;
BorderLayout borderLayout1 = new BorderLayout();
JPanel contentPane = new JPanel();
JTextField result = new JTextField("");
double stackedValue = 0.0;
boolean isStacked = false;
boolean afterCalc = false;
String currentOp = "";

  public static void main(String args[]){
 CalcSample frame = new CalcSample("Calculator");
 frame.setVisible(true);
 }

  CalcSample(String title){
 contentPane.setLayout(borderLayout1);
 setTitle(title);
 setSize(250, 350);
 setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
 setContentPane(contentPane);
 
 contentPane.add(result, BorderLayout.NORTH);

 JPanel keyPanel = new JPanel();
 keyPanel.setLayout(new GridLayout(4, 4));
 contentPane.add(keyPanel, BorderLayout.CENTER);

 keyPanel.add(new NumberButton("7"), 0);
 keyPanel.add(new NumberButton("8"), 1);
 keyPanel.add(new NumberButton("9"), 2);
 keyPanel.add(new CalcButton("÷"), 3);
 keyPanel.add(new NumberButton("4"), 4);
 keyPanel.add(new NumberButton("5"), 5);
 keyPanel.add(new NumberButton("6"), 6);
 keyPanel.add(new CalcButton("×"), 7);
 keyPanel.add(new NumberButton("1"), 8);
 keyPanel.add(new NumberButton("2"), 9);
 keyPanel.add(new NumberButton("3"), 10);
 keyPanel.add(new CalcButton("-"), 11);
 keyPanel.add(new NumberButton("0"), 12);
 keyPanel.add(new CalcButton("."), 13);
 keyPanel.add(new CalcButton("+"), 14);
 keyPanel.add(new CalcButton("="), 15);

 contentPane.add(new ClearButton(), BorderLayout.SOUTH);
   }


 public void appendResult(String c) {
  if (!afterCalc)
   result.setText(result.getText() + c);
  else {
   result.setText(c);
   afterCalc = false;
  }
 }

 
 public class NumberButton extends JButton implements ActionListener {
  private static final long serialVersionUID = 1L;

  public NumberButton(String keyTop) {
   super(keyTop);
   this.addActionListener(this);
  }

  public void actionPerformed(ActionEvent evt) {
   String keyNumber = this.getText();
   appendResult(keyNumber);
  }
 }

 
 public class CalcButton extends JButton implements ActionListener {
  private static final long serialVersionUID = 1L;

  public CalcButton(String op) {
   super(op);
   this.addActionListener(this);
  }

  public void actionPerformed(ActionEvent e) {
   if (isStacked) {
    double resultValue = (Double.valueOf(result.getText()))
      .doubleValue();
    if (currentOp.equals("+"))
     stackedValue += resultValue;
    else if (currentOp.equals("-"))
     stackedValue -= resultValue;
    else if (currentOp.equals("×"))
     stackedValue *= resultValue;
    else if (currentOp.equals("÷"))
     stackedValue /= resultValue;
    result.setText(String.valueOf(stackedValue));
   }
   currentOp = this.getText();
   stackedValue = (Double.valueOf(result.getText())).doubleValue();
   afterCalc = true;
   if (currentOp.equals("="))
    isStacked = false;
   else
    isStacked = true;
  }
 }

 
 public class ClearButton extends JButton implements ActionListener {

  private static final long serialVersionUID = 1L;

  public ClearButton() {
   super("C");
   this.addActionListener(this);
  }

  public void actionPerformed(ActionEvent evt) {
   stackedValue = 0.0;
   afterCalc = false;
   isStacked = false;
   result.setText("");
  }
 }
}